Efficient light coupler from off-chip to on-chip waveguides

ABSTRACT

In an embodiment, light from a single mode light source may be deflected into a low index contrast (LIC) waveguide in an opto-electronic integrated circuit (OEIC) (or “opto-electronic chip”) by a 45 degree mirror. The mirror may be formed by polishing an edge of the die at a 45 degree angle and coating the polished edge with a metal layer. Light coupled into the LIC waveguide may then be transferred from the LIC waveguide to a high index contrast (HIC) waveguide by evanescent coupling.

BACKGROUND

[0001] Opto-electronic integrated circuits (OEICS) may incorporate bothelectronic circuits and optical devices, such as integrated waveguides,modulators, switches, and detectors. The optical devices may be usedfor, e.g., optical clock distribution, intra-chip optical signaling, andchip-to-chip communication Both the electronic circuits and opticaldevices may be produced on silicon using complementary metal-oxidesemiconductor (CMOS) fabrication techniques.

[0002] Light utilized by optical devices in an OEIC may be introducedinto the chip by an external source, such as a vertical cavity surfaceemitting laser (VCSEL) or an optical fiber. The light from the externalsource may have a relatively large mode compared to that of the on-chipwaveguides. The differences in mode size may present difficulties inefficiently coupling the relatively large mode off-chip light source toa small waveguide on the chip.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003]FIG. 1 is a cross-sectional view of an opto-electronic chip bondedto a flip chip.

[0004]FIG. 2 is a plan view of an optical layer in the opto-electronicchip.

[0005]FIG. 3 is a sectional view of an integrated waveguide structure.

[0006]FIG. 4 is a plan view of an integrated waveguide structure.

[0007]FIG. 5 is a sectional view of adjacent low and high index contrastwaveguides.

[0008] FIG. G is a chart showing the normalized propagation constant asa function of the separation of the waveguides of FIG. 5.

[0009]FIGS. 7A and 7B are plots showing the localization of thedispersion curve in the waveguides of FIG. 5.

[0010]FIGS. 8A-8J show steps in an exemplary process for fabricating theopto-electronic chip shown in FIG. 1.

[0011]FIG. 9 shows an opto-electronic chip according to an alternativeembodiment.

DETAILED DESCRIPTION

[0012]FIG. 1 shows an opto-electronic integrated circuit (OEIC) (or“opto-electronic chip”) 100 coupled to a flip chip package 105. The flipchip package may include a light source 110, e.g., a laser or opticalfiber. Modulated light signals from the light source may be deflectedinto a low index contrast (LIC) waveguide 115 by a 45 degree mirror 118.The LIC waveguide may be mode-matched to the light source 110 tominimize coupling loss. Light coupled into the LIC waveguide 110 maythen be transferred from the LIC waveguide to a high index contrast(HIC) waveguide 120 by evanescent coupling.

[0013] The HIC waveguide 120 may be laid out in a pattern, e.g., a treestructure, to distribute the light across the chip, as shown in FIG. 2.Photodetectors 125 may convert the light signals into electricalsignals. The electrical signals may be transferred to electroniccircuitry in the chip through electrical interconnects 130 inmetallization layers 135 of the chip 100.

[0014] The light source may be a single mode (SM) optical fiber, VCSEL(Vertical Cavity Surface Emitting Laser), or other single modesemiconductor laser. “Mode” refers to the solution of Maxwell's waveequation satisfying the boundary conditions of the waveguide, thusforming a unique pattern of standing wave in the radial direction on thecross section of the waveguide. A mode is characterized by itspropagation constant (eigenvalue of the wave equation). A single modelight source may be appropriate for the relatively small waveguidespresent in the opto-electronic chip.

[0015] The light source may be positioned vertically with respect to thedevice side of the chip and placed in close proximity. The light mayimpinge on the surface of the chip and be transmitted through atransparent cladding film 150 (e.g., SiO₂) and across the LIC waveguidematerial 115. Anti-reflective (AR) coatings may be provided on the chipsurface to avoid reflection.

[0016] The light may then strike a 45 degree metal mirror and bereflected 90 degrees, in the same direction as the waveguide, i.e.,parallel to the chip surface. The light may be trapped by total internalreflection and coupled into the LIC waveguide 115. The index contrast ofthis waveguide (e.g., the difference between the indexes of refractionof the waveguide core and the surrounding cladding layer) may betailored such that the mode size is close to that of the fiber topromote efficient coupling, thereby reducing the power requirement forthe off-chip light source.

[0017] As shown in FIGS. 1 and 2, the LIC waveguide 115 may be largerthan the HIC waveguide 120. The mode of the LIC waveguide 115 may moreclosely match the mode of the light source 110. However, the bend radiiof HIC waveguides may be much smaller (e.g., less than about 50 microns)compared to LIC waveguides, which may be only able to bend at about 1 mmradius. Having a smaller alloable bend radius allows for more efficientdistribution of light about the chip. Accordingly, the LIC waveguide 115may be used to couple light into the chip, and the HIC waveguide(s) 120may be used for distribution and signaling.

[0018] A cross section and a top view of an integrated waveguide areshown in FIGS. 3 and 4, respectively. The waveguide may be an opticallyguiding core 305 of a material with refractive index n_(w) surrounded bya cladding material with a different index of refraction, n_(c). Thehigh contrast of the refractive index between the two materials confinesa lightwave to the waveguide 305. The cladding material may be, e.g.,silicon oxide (SiO₂) (n_(c)≈1.5). The waveguide material may be selectedfrom, e.g., silicon nitride (Si₃N₄) (n_(w)≈2), silicon (Si) (n_(w)≈3),and silicon oxynitride (SiON) (n_(w)≈1.55). Silicon oxynitride may offerdesign flexibility because its refractive index may be varied bychanging the content of nitrogen. The difference in the indexes ofrefraction between the core and the cladding determines the contrast,e.g., high index contrast or low index contrast.

[0019] Light may be transferred from the LIC waveguide 115 to the HICwaveguide 120 by evanescent coupling. Since the index of the HICwaveguide 120 is higher than that of the LIC waveguide 115, the lightgets coupled through the evanescent tail of the low index contrast mode.A lithographically patterned taper 200 may be used at the end of the LICwaveguide to make the transfer occur over a shorter length, as shown inFIG. 2. The interaction length may be designed such that substantiallyall of the light is transferred to the lower HIC waveguide 120.

[0020]FIG. 5 shows two single mode waveguides 505, 510 with n_(w1)=1.6(LIC) and n_(w2)=2.0 (HIC), respectively, and cladding index n_(c)=1.5.The distance “d” may be varied from 0.0 to 1.2 microns. FIGS. 6 and 7A-Billustrate modeling simulations for this waveguide configuration. FIG. 6is a chart illustrating the normalized propagation constant as afunction of the separation of waveguides. Each waveguide contains adoubly degenerate effective index when isolated. The upper branch ofdispersion may be asymmetric mode localized to the HIC waveguide 510(modes 0 and 1) and the lower branch corresponds to LIC waveguide (modes2 and 3). FIG. 7A shows that the upper branch of the dispersion curve islocalized in the HIC waveguide and is weakly coupled to the LICwaveguide. FIG. 7B shows that the lower branch of the dispersion curveis localized in LIC waveguide and strongly coupled to HIC waveguide. Thecoupling efficiency ranges from 70% at d=0.0 to 20% at d=0.5 based onthe ratio of peak amplitudes in waveguides.

[0021] FIGS. 8A-J show stages in the fabrication of the optical layersand 45 degree mirror in the chip according to an embodiment. A lowercladding film 800, such as SiO₂ may be deposited on the top of the lastmetallization layer 805 in the chip, which may include electricalinterconnect lines to electronic circuitry in the chip. A core material810 for the HIC waveguide 120, such as Si₃N₄, may be deposited on thelower cladding film 800. The silicon nitride layer may then be etched toform a HIC waveguide pattern. An intermediate cladding layer 815, e.g.,silicon dioxide, may be deposited over the HIC waveguide layer 810.Next, a core material 820 for the LIC waveguide 115 may be deposited onthe intermediate cladding layer 815. The silicon oxynitride layer maythen be etched to form a LIC waveguide pattern. An upper cladding layer825 may then be deposited on the LIC waveguide pattern.

[0022] The wafer may then be diced, producing an edge 830. The edge 830of the die may be polished to a 45 degree angle edge 835. A thin layer840 of a metal material such as Al may be applied, e.g., by sputteringor evaporation. Anti-reflective coatings may also be added to the topsurface to reduce reflection. The light source, e.g., an optical fiber,may then be joined to the top of the upper cladding layer 825 of the LICwaveguide, e.g., melting the fiber to adhere to the surface or by use ofan adhesive.

[0023] In another embodiment, light may enter the backside surface ofthe chip and hit a mirror 905 which is polished at 45 degrees, as inFIG. 9. This structure may be used with light having a wavelengthgreater than about 1.2 microns.

[0024] A number of embodiments have been described. Nevertheless, itwill be understood that various modifications may be made withoutdeparting from the spirit and scope of the invention. Accordingly, otherembodiments are within the scope of the following claims.

1. An apparatus comprising: a die including a waveguide substantiallyparallel to a surface of the die, and a mirror to couple light directedsubstantially parallel to the surface of the die into the waveguide, themirror including a polished edge of the die covered with a reflectivematerial.
 2. The apparatus of claim 1, wherein the waveguide comprises alow index contrast waveguide.
 3. The apparatus of claim 2, furthercomprising a high index contrast waveguide adjacent the low indexcontrast waveguide.
 4. The apparatus of claim 3, wherein the high indexcontrast waveguide is spaced apart from the low index contrast waveguidea distance operative to couple light from the low index contrastwaveguide into the high index contrast waveguide.
 5. The apparatus ofclaim 3, further comprising photodetectors operative to convert light inthe high index contrast waveguide into an electrical signal.
 6. Theapparatus of claim 3, wherein the high index contrast waveguidecomprises a pattern of waveguides operative to distribute light acrossthe die.
 7. The apparatus of claim 3, wherein the low index contrastwaveguide comprises a tapered section at a region adjacent to the highindex contrast waveguide.
 8. The apparatus of claim 1, wherein the lightcomprises single mode light.
 9. The apparatus of claim 1, furthercomprising a plurality of layers including electronic circuitry.
 10. Theapparatus of claim 1, wherein the polished edge of the die is at anangle of approximately 45 degrees from the surface of the die.
 11. Theapparatus of claim 1, wherein the reflective material comprises a layerof metal.
 12. The apparatus of claim 1, wherein the surface of the dieincludes an anti-reflective coating.
 13. A method comprising: forming awaveguide in a semiconductor die substantially parallel to a surface ofthe die; polishing an edge of the die at an angle; and coating thepolished edge of the die with a reflective material to form a mirroroperative to couple light directed substantially normal to the diesurface into the waveguide.
 14. The method of claim 13, wherein saidforming the waveguide comprises forming a low index contrast waveguide.15. The method of claim 14, further comprising forming a high indexcontrast waveguide adjacent to the low index contrast waveguide at adistance operative to couple light in the low index contrast waveguideinto the high index contrast waveguide.
 16. The method of claim 14,further comprising patterning the high index contrast waveguide into adistribution pattern on the die.
 17. The method of claim 13, furthercomprising coating the surface of the die with an anti-reflectivecoating.
 18. The method of claim 13, further comprising coupling a lightsource to the die such that light from the source is directed into thedie at an angle substantially perpendicular to the die surface.
 19. Themethod of claim 13, wherein said polishing comprises polishing the edgeof the die on an angle of approximately 45 degrees from the die surface.20. A system comprising: a die including a waveguide substantiallyparallel to a surface of the die, and a mirror to couple light directedsubstantially parallel to the surface into the waveguide, the mirrorincluding a polished edge of the die covered with a reflective material;and a light source coupled to the die and operative to direct a lightbeam into the die at an angle substantially perpendicular to the diesurface.
 21. The system of claim 20, wherein the waveguide comprises alow index contrast waveguide.
 22. The system of claim 21, furthercomprising a high index contrast waveguide adjacent the low indexcontrast waveguide.
 23. The system of claim 22, wherein the high indexcontrast waveguide is spaced apart from the low index contrast waveguidea distance operative to couple light from the low index contrastwaveguide into the high index contrast waveguide.
 24. The system ofclaim 20, wherein the light source comprises single mode light source.25. The system of claim 20, further comprising a plurality of layersincluding electronic circuitry.
 26. The system of claim 20, wherein thepolished edge of the die is at an angle of approximately 45 degrees fromthe surface of the die.